1. Field of the Invention
The present invention relates to an electronic camera for producing an image having a wide dynamic range by composing or synthesizing two image signals obtained by picking-up an object two times with a desired exposure amount ratio with the aid of a single solid state image sensing element, and more particularly to an electronic camera for producing a composed image having a wide dynamic range as well as a high quality by utilizing a flash light emitting device, e.g. stroboscopic flash device. Such an electronic camera is generally called a filmless digital still camera.
2. Description of the Related Art
In the electronic camera, use is generally made of a solid state image sensing element such as CCD image sensing element. It is well known that such a solid state image sensing element is liable to be saturated with a relatively low luminance and has a low S/N for a lower luminance. Therefore, there is a problem that a dynamic range of the solid state image sensing element is very narrower than a conventional photographic film.
In order to solve such a problem, there has been proposed an electronic camera, in which an image having an extended dynamic range is obtained by composing two image signals which are produced by a single solid state image sensing element when an object is picked-up with different exposure amounts two times successively. For instance, in Japanese Patent Application Laid-open Publication Kokai Hei 4-207581, there is disclosed an electronic camera, in which immediately after transferring signal charges stored in a light receiving section of a solid state image sensing element during a first exposure into a transfer section of the element, a second exposure having a longer exposure time than the first exposure is initiated, and signal charges stored in the light receiving section during the second exposure are transferred into the transfer section after the signal charges of the first exposure have been outputted from the solid state image sensing element. In this manner, the second exposure can be performed for a time interval during which the signal charges produced by the first exposure and transferred into the vertical transfer section are read out of the solid state image sensing element, and therefore it is possible to obtain two image signals with a short time interval. Then, the thus obtained two image signals are composed to derive a composed image having a wider dynamic range.
Further, in Japanese Patent Application Laid-open Publication Kokai Sho 62-108678, there is disclosed an electronic camera, in which upon producing an image with a wide dynamic range by composing two image signals obtained with different exposure amounts, one of the image signals is obtained by picking-up an object under the natural light, and the other image signal is obtained by picking-up the same object with a stroboscopic flash light, and these image signals are compared with each other to extract image portions having a larger amplitude to obtain a composed image.
In general, when an object has a lower luminance level, the image is picked-up with stroboscopic flash light. However, in case of obtaining an image with a wide dynamic range by composing two image signals obtained with different exposure amounts, when one of the image signals is obtained with the natural light and the other image signal is obtained with the flash light and the thus obtained two image signals are merely composed, it is difficult to obtain the two image signals with a desired exposure amount ratio in a highly precise manner although exposure times are set accurately owing to the fact that stroboscopic flash light fluctuates largely.
Furthermore, when the stroboscopic flash light is used, a portion of an object which is irradiated with the stroboscopic flash light can be picked-up such that the image signal with the stroboscopic flash light can have the given exposure amount ration with respect to the image signal with the natural light, but at apportion of the object which is not irradiated with the stroboscopic flash light, only the image signal with a same exposure amount as that with the natural light is obtained by the second exposure, and therefore there is a problem that the image picking-up operations could not be conducted with a desired exposure amount ratio between the first and second shots. Moreover, since a white balance of the image signal with the natural light differs from that with the stroboscopic flash light, when the image signals are composed, a color balance might be partially lost.
In order to solve the above mentioned problems, there has been proposed an electronic camera, in which first and second image picking-up operations are carried out successively with stroboscopic flash light to produce two image signals, and the thus obtained two image signals are synthesized to form a composed image. When both the two image signals to be composed are derived with the stroboscopic flash light, it is easy to perform the first and second image picking-up operations with a desired exposure amount ratio over a whole image area, and therefore it is possible to obtain a composed image having a wide dynamic range and having a uniform white balance over a whole image.
In this case, there may be provided two sets of stroboscopic flash devices each comprising a Xenon tube, a trigger capacitance and a light emission capacitance, these stroboscopic flash devices are alternately used for the first and second exposures. However, it is apparent that the electronic camera including such a system is liable to be high in cost, large in size and heavy in weight. Therefore, it is desired that the two image picking-up operations are performed by means of a single stroboscopic flash device which is used in a conventional camera in which no image signal composition is carried out.
In this case, when the stroboscopic flash device is operated for the two exposures like as the conventional stroboscopic flash exposure, i.e. when the trigger capacitance is charged and a trigger voltage is applied to the Xenon tube each time the stroboscopic flash exposure is carried out, two image signals could not be obtained with a short time interval. This is due to the fact that in general, the trigger capacitance is charged through a resistor, and therefore a relatively long time period such as 6 msec is required for the trigger capacitance due to a time constant of the triggering circuit until the trigger capacitance is charged up to an effective trigger voltage. This results in that the second exposure could not be initiated immediately after the completion of the first exposure. In this manner, a relatively long time interval is required from the end of the first exposure to the start of the second exposure. This affects the second exposure and its shutter speed, and particularly when the object includes a moving subject, a portion of the composed image corresponding the relative movement might be deteriorated. In case of the color image, false color or false contour might be produced.
Moreover, when the second exposure is performed by applying the electrostatic charge stored in the trigger capacitance as the trigger voltage to the Xenon tube like as the conventional stroboscopic flash exposure, the trigger voltage having an amplitude of several KVs might produce undesired noise, and if this second triggering period overlaps with the storing period or transferring period of the signal charges of the first exposure, the image signal obtained with the first exposure might be deteriorated.
The present invention has for its object to provide a novel and useful electronic camera, which can solve the above mentioned problems and can produce a composed image having a high quality and a wide dynamic range by conducting first and second exposures with flash light.
According to the invention, an electronic camera producing an image having a wide dynamic range by composing two image signals obtained by picking-up an object two times successively with different exposure amounts, comprises:
an image picking-up means including a solid state image sensing element for picking-up an object to derive an image signal;
a flash light emitting means for irradiating said object with flash light;
an image signal composing means for composing two image signals obtained by picking-up the object two times with a desired exposure amount ratio; and
a flash light emission controlling means for controlling an excited emission of said flash light emitting means in accordance with said desired exposure amount ratio such that upon the first exposure, said flash light emitting means is excited and the flash light is emitted, an excited condition of the flash light emitting means is kept over said second exposure, and after once stopping the light emission at an end of the first exposure, the flash light is emitted again to perform the second exposure.
In the electronic camera according to the invention, said flash light emitting means is constituted by a conventional stroboscopic flash device including a discharge tube such as a Xenon tube, and the discharge tube is kept in the excited condition from the start of the first exposure to the end of the second exposure. Therefore, it is no more necessary to recharge a trigger capacitance between the first exposure and the second exposure, and thus the two exposures can be carried out with the desired exposure amount ratio, while a time interval between the first exposure and the second exposure can be shortened. Therefore, even if the object including a moving subject, it is always possible to obtain a composed image having a high quality.
In an embodiment of the electronic camera according to the invention, said image picking-up means is constructed such that electrostatic charges stored in a light receiving section of the solid state image sensing element during the first exposure are transferred into a vertical transfer section of the solid state image sensing element during the time interval between the first exposure and the second exposure during which the excited light emission is stopped. In this embodiment, the first image signal is no more affected by the excited light emission for the second exposure, and therefore it is possible to attain the image signal having a high S/N, and the time interval during which the excited light emission is interrupted can be further shortened.
In another preferable embodiment of the electronic camera according to the invention, said flash light emission controlling means includes an exposure control table which represents a plurality of exposure control data values for the first exposure and a plurality of exposure control data values for the second exposure, said exposure control data values determining excited light emission times for the first and second exposures in accordance with said desired exposure amount ratio, and the excited light emission is controlled by selecting, from said exposure control table, exposure control data values corresponding to an image picking-up condition.
Then, the first and second image picking-up operations can be performed accurately in accordance with the desired exposure amount ratio by selecting exposure control data values in accordance with a image picking-up condition for attaining correct exposure. In this embodiment, it is preferable that said flash light emitting means comprises a flash light source, a trigger capacitance for driving said flash light source into an excited condition, and an light emission capacitance for supplying a light emitting energy to said flash light source in the excited condition to emit an excited light. The exposure control data values for the first exposure in said exposure control table of said flash light emission controlling means are determined on the basis of a reference voltage of said light emission capacitance, and the exposure control data values for the second exposure are determined on the basis of a voltage across said light emission capacitance which is determined by considering a voltage drop due the excited light emission for the first exposure. In this embodiment, it is possible to attain much more accurate exposure control data values corresponding to the predetermined exposure amount ratio.
Alternatively, said plurality of exposure control data values of the first and second exposures may be constructed by exposure control data values for a conventional single exposure in which no image composition is carried out. In such a case, a memory for storing the exposure control data values can have a smaller capacity, and the whole camera can be cheap in cost.
Furthermore, in the above preferable embodiment, said flash light emission controlling means includes a voltage detecting means for detecting a voltage across said light emission capacitance, and the excited light emission is controlled by selecting, from said exposure control table, an exposure control data value for the first exposure and an exposure control data value for the second exposure suitable for an exposure condition on the basis of a comparison of the detected voltage with the reference voltage. In this case, in the first exposure, even if the light emission capacitance is not fully charged up to the reference voltage, it is possible to attain much more accurate exposure control data values corresponding to the desired exposure amount ratio.
In the electronic camera according to the invention, said image picking-up means includes a variable stop which is controlled in accordance with a distance from the camera to the object, and exposure control data values for the first and second exposure control data values are selected from said exposure control table on the basis of a figure of the variable stop, i.e. stop value. In this case, for instance, when a distance to the object is short, the stop value may be increased to extend a workable range to a shorter distance side, and the image pick-up with a short distance, i.e. close-up shot can be easily realized.
Furthermore, in the electronic camera according to the invention, said flash light emitting means is controlled by said flash light emission controlling means such that flat light is emitted from said flash light emitting means in each of said first and second exposures. In this manner, during respective exposures, the flash light emitting means is operated to emit the excited light in a pulsatory manner with a short time non-emission interval, and therefore exposure amounts can be controlled much more accurately and it is possible to attain the image having a much higher quality. It should be noted that the light emission interruption period between the first exposure and the second exposure according to the invention does not mean the very short time non-emission interval in such a pulsatory light emission, and both terms should not be confused.
It should be noted that according to the invention, said exposure amount ratio between the first exposure and the second exposure may be previously determined or may be determined in accordance with an image pick-up condition.